This retrospective study reviewed incident reports to characterize diagnostic errors occurring in emergency departments in England and Wales. The majority of incidents (86%) were delayed diagnoses; the remainder were wrong diagnoses. The authors identified three themes stemming from human factors that contributed to the diagnostic errors: insufficient assessment (e.g., failure to order imaging or refer patients when indicated), inappropriate response to diagnostic imaging, and failure to order diagnostic imaging. Potential interventions to address these contributors are briefly discussed.